IntervalIntegrate.frink

// This function takes an anonymous one-argument function f and numerically// integrates it over n steps from lower to upper. The result is an interval// that should be guaranted to contain the true value of the function.
IntervalIntegrate[f, lower, upper, steps] :=
{// Make sure steps is an integer or this will fail.
steps = ceil[steps]
stepsize = (upper-lower) / steps